This invention relates to a novel matrix resin, and more particularly, to a bis-maleimide resin and high temperature, high performance fiber composites thereof.
Bis-maleimide (BMI) materials are based on the condensation reaction of maleic anhydride with amines, usually diamines, to yield a fully imidized material that can crosslink without the evolution of volatiles to produce a structural matrix. The crosslinking is typically accomplished by means of homopolymerization of the unsaturated maleimide end cap and by Michael addition utilizing additional amine as a curing agent. The resultant cured resin has an intermediate thermal stability between the epoxies and the high temperature polyimide (PI) materials. Thermal performance is good to about 500.degree. F., but not above, and thermo-oxidative stability is less than the high temperature PI's. The major advantage of BMI resins is epoxy-like processing without the evolution of volatiles in cure so that very low void content composites can be produced. Additionally, the systems are usually formulated to cure in the 350.degree.-400.degree. F. region under moderate pressure which means that conventional process equipment, autoclaves and presses, and conventional vacuum bagging materials can be used. Of the high temperature PI's, only the condensation types are processable under these conditions, and these have the major drawback of high void content in the finished structure.
Some of these materials are described in U.S. Pat. No. 3,380,964. Typically, three basic compositions comprise: (i) those which contain a single BMI base resin, e.g., methylene bisphenyl maleimide, and an aromatic diamine, e.g., methylene dianiline (MDA); (ii) those which contain a single BMI resin without diamine; and (iii) those which comprise a eutectic blend of three BMI base resins including at least one with an aliphatic backbone. All of these materials are solids at ambient. The latter two are relatively low melting materials, and the last-mentioned is an extemely low viscosity and low reactivity material containing neither free diamine nor catalyst. The first-mentioned is the highest viscosity member of this group, may be prereacted, and has relatively high reactivity at 350.degree. F.
In the present state of the art, BMI materials are being selected over epoxies due to increased 350.degree.-400.degree. F. performance, particularly following hot/wet conditioning. A major use of graphite/BMI is for supersonic fighter wing skins. They are also being used in helicopter parts. Another use is for very long term 350.degree. F. nacelle stuctures (30,000 hours). Interest has also been expressed in BMI materials for other aerospace applications. The best positioned material to date is a system embodied in U.S. Pat. No. 4,454,283 which is a eutectic BMI blend with divinyl benzene as a reactive tackifier. A curing agent or catalyst is also present, either in the base resin or added to it. This is the best current BMI with tack and drape for mechanical properties and thermo-oxidative stability, but it still has drawbacks. Among these is a strong odor, potential handling toxicity, and still insufficient oxidative stability. This BMI system is better than others on graphite in that apparently less microcacking is observed in composites made with it.
Another state-of-the-art system is described in U.S. Pat. No. 4,100,140, the BMI resin being cured with an alkenyl phenol or alkenylphenol ether. While such compositions provide laminated structures with better impact performance than the BMI eutectic blend with divinyl benzene, their hot strength properties are not as good, and after exposure to moisture, the hot strength properties are extremely poor.
In the present state-of-the-art, therefore, problems remaining to be resolved for the successful development of a structural BMI product are cure shrinkage (and microcracking), handleability, handling toxicity, ease of cure, and environmental stability--thermo-oxidative and hydrothermal. Such a resin composition can offer a great potential for even wider use if toughness can be incorporated in addition to the thermal properties, and such a resin composition is provided by the present invention.